I. Field of the Invention
The present invention relates generally to spread spectrum communication systems, and more particularly, to a method and apparatus for adjusting signal parameters in such systems in the presence of control loop or path delay in detecting signal status and using a controllable element to effect changes in the detected status. The invention further relates to using transmit power as a parameter that is controlled to minimize interference among simultaneously operating transmitters and to maximize the quality of individual communications.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters," and U.S. patent application Ser. No. 08/368,570, entitled "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," both of which are assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which a large number of generally mobile or remote system users or subscriber units ("mobile units") employ at least one transceiver to communicate with other mobile units, or users of other connected systems, such as a public telephone switching network. Communication signals are transferred either through satellite repeaters and gateways, or directly to terrestrial base stations (also sometimes referred to as cell-sites or cells).
In CDMA communications, the frequency spectrum can be reused multiple times, thereby permitting an increase in the number of mobile units. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques. However, in order to maximize overall communication system capacity and maintain acceptable levels of mutual interference and signal quality, the transmitted power of signals within the system must be controlled so as to maintain, at a minimum level, the amount of power required for any given communication link. By controlling the transmitted signal power at or near the minimum level, interference with other mobile units is reduced.
In communication systems employing satellites, the communication signals typically experience fading that is characterized as Rician. Accordingly, the received signal consists of a direct component summed with multiple reflected components having Rayleigh fading statistics. The power ratio between the direct component and the reflected components is typically on the order of 6-10 dB, depending upon the characteristics of the mobile unit antenna and the environment in which the mobile unit operates.
In contrast to satellite communication systems, communication signals in terrestrial communication systems typically experience signal fading that typically consists only of the reflected, or Rayleigh, components, without a direct component. Thus, the terrestrial communication signals experience a more severe fading environment than the satellite communication signals where Rician fading is the dominant fading characteristic.
The Rayleigh fading in the terrestrial communication system is caused by the communication signals being reflected from many different features of the physical environment. As a result, a signal arrives almost simultaneously at a mobile unit receiver from many directions with different transmission delays. At the UHF frequency bands usually employed by mobile radio communications including those of cellular mobile telephone systems, significant phase differences in signals traveling on different paths may occur. The possibility for destructive summation of the signals may result in occasional deep fades.
In order to provide a full-duplex channel to allow both directions of a conversation to be simultaneously active, such as provided by the conventional wired telephone system, one frequency band is used for an outbound or forward link, (i.e. transmission from the gateway or cell-site transmitter to the mobile unit receiver), and a different frequency band is utilized for the inbound or reverse link, (i.e. transmission from the mobile unit transmitter to the gateway or cell-site receiver). This frequency band separation allows a mobile unit transmitter and receiver to be active simultaneously without feedback or interference from the transmitter into the receiver.
However, using different frequency bands has significant implications for power control. Using different frequency bands causes multipath fading to be independent processes for the forward and reverse links. Forward link path loss cannot simply be measured and have it assumed that the same path loss is present on the reverse link.
Furthermore, in a cellular mobile telephone system the mobile phone is capable of communications through multiple cell-sites as disclosed in copending U.S. patent application Ser. No. 07/433,030, filed Nov. 7, 1989 entitled "Method And System For Providing A Soft Handoff In Communications In A CDMA Cellular Telephone System," the disclosure of which is incorporated herein by reference. In communications with multiple cell-sites, the mobile unit and cell-sites include a multiple receiver scheme as disclosed in the just mentioned application and further detailed in copending U.S. patent application Ser. No. 07/432,552, also filed Nov. 7, 1989 and entitled "Diversity Receiver In A CDMA Cellular Telephone System," the disclosure of which is also incorporated herein by reference.
One method of power control is to have either the mobile unit or the gateway first measure the power level of a received signal. This power measurement is used, along with a knowledge of transponder downlink transmit power levels for each satellite being used and knowledge of mobile unit and gateway receiver sensitivity, to estimate path loss for each channel of the mobile unit. Either the base station or the mobile unit transceiver can then determine the appropriate power to be used for signal transmissions to the mobile unit, taking into account the path loss estimate, a transmitted data rate, and a satellite receiver sensitivity. In the case of the mobile unit, a request can be made for more or less power in response to such measurements and determinations. At the same time, the gateway can increase or decrease power in response to such requests, or in response to its own measurements.
The signals transmitted by the mobile unit to the satellite are relayed by the satellite to the gateway and generally on to a communication system control system. The gateway or the control system measures the received signal power from the transmitted signals. The gateway then determines the deviation in the received power level from a minimum which is necessary to maintain the desired level of communications. Preferably, the minimum desired power level is that power level necessary to maintain quality communications while reducing system interference.
The gateway then transmits a power control command signal to the mobile unit so as to adjust or "fine tune" the transmit power of the mobile unit. This command signal is used by the mobile unit to change the transmit power level closer to the minimum level required to maintain the desired communications. As channel conditions change, typically due to motion of the mobile unit, or satellite, the mobile unit responds to the control commands from the gateway to continually adjust the transmit power level so as to maintain a proper power level.
In this configuration, the control commands from the gateway are referred to as power control feedback. The power control feedback from the gateway is generally quite slow due to round trip propagation delays through the satellites. A one way propagation delay employing a typical LEO satellite orbit (879 miles) is on the order of 9-26 ms. Thus, a power control command from the gateway can reach the mobile unit up to 26 ms after it was sent. Likewise, a change in the transmitted power made by the mobile unit in response to the power control command is detected by the gateway up to 26 ms after the change was made. The total round-trip propagation delay in this system is on the order of 18-53 ms. Thus, up to 53 ms of delay may elapse between the time a power control command is sent by the gateway and the time the response (i.e., the change in the power level caused by that power control command) is detected back at the gateway.
Thus, a transmit power control command experiences the round trip propagation delay, as well as typical processing delays, before the results of that command can be detected by the measuring unit. Unfortunately, particularly where the propagation delay is large, an adjustment to the transmit power in response to the power control command made by the mobile unit will not occur and be detected by the gateway before the next time the received power is measured at the gateway. This results in another power control command being sent to adjust the transmit power without the benefit of the previous power control command having been implemented. In fact, depending on the propagation delay and the iteration time of the power control loop, several power control commands may be pending or "propagating" before the first power control command is responded to by the mobile unit and the results detected by the gateway. As a result, the transmit power oscillates about a set point in what is referred to as a "limit cycle." That is, the transmit power over- or undershoots from a desired amount due to delays in arrival and implementation of commands.
One possible solution to this problem is to simply increase the iteration time of the power control loop so that it more closely resembles the propagation and processing delays. However, the impact of rapid fading and sudden signal blockages experienced by the communication signals require short iteration times to prevent sudden signal loss. As a result, the transmit power may suddenly, and unnecessarily, be increased, resulting in wasted power and increased system interference.
What is needed is a method and an apparatus that quickly responds to changes in transmit signal power, or other signal parameters, requirements, and counteracts the impact of propagation and processing delays associated with corresponding control commands. It is desirable that such a method and apparatus require little additional complexity, control structure, or protocol changes in the gateways.